1. Field of the Invention
This invention relates to the application of coded information to objects, which coded information is subsequently electronically read. The invention relates to coding systems not visible to the human eye, which are particularly useful in reprographic machines, printers and other document handling equipment.
2. Related Developments
Bar code reading systems in which bar codes on an object are invisible to the human eye but can be read electronically have a wide area of application. Such systems may be used in office document duplicators to code documents for security purposes to prevent unauthorized copies from being made. These coding systems may also be used to code documents for the control of various operations within the duplicator, such as sorting and paper path selection, as described in U.S. Pat. Nos. 4,716,438 and 4,757,348, the disclosures of which are incorporated herein by reference. Information necessary to provide color correction, enhancement and translation, document identification, image preservation, and document control and security can also be provided in taggants that are incorporated into marking materials used to create images or codes, as described in U.S. Pat. No. 5,225,900 (the disclosure of which is incorporated herein by reference), assigned to the assignee of the present invention.
Invisible bar codes systems also find use on commercial product labeling. For this use, the invisible bar code system has several principal advantages over visible bar code systems. The product can have a bar code placed on all sides of the package, thereby increasing the convenience of entering the bar code into a bar code reader without reorienting the package. Invisible bar codes do not detract from the appearance of consumer products, such as perfumes or magazines. Additional bar code information can be placed on the product, such as expiration dating and additional manufacturers' identification number, which are not evident to the purchaser or user without special decoding equipment. Invisible bar codes can be used to mark mail without obliterating markings that are already present.
Invisible bar codes systems utilizing ultraviolet (UV) stimulated visible fluorescent dyes in the bar code have been proposed. However, these systems have two principal difficulties. UV light used to stimulate the fluorescent dyes of the bar codes also stimulates fluorescence of paper whiteners commonly used in paper stock, thereby making readout difficult. If the bar code is placed over underlaying print, the signal is deteriorated because printing inks tend to absorb the incident and fluorescent radiation, thereby rendering detection even more difficult. Further, inexpensive, compact and concentrated UV light sources are not readily commercially available.
Systems for overcoming the disadvantages of UV stimulated invisible bar codes have involved the use of infrared (IR) fluorescent bar codes that are stimulated in the visible or near IR spectrum and fluoresce at longer wavelengths in the IR spectrum. Such systems are described in the paper, A Novel Bar Coding System for Non Letter Mail by T. Dolash, P. Andrus and L. Stockum, presented at the 3rd Advanced Technology Conference, Washington, D.C., May 3-5, 1988, in U.S. Pat. No. 4,983,817 and in U.S. Pat. No. 5,093,147. In these systems, a bar code containing an IR fluorescent dye is scanned with stimulating radiation from a red helium-neon laser or a gallium-aluminum-arsenide laser to activate the IR fluorescence of the dye. This signal is then detected with an IR photodiode. These systems have advantages over those utilizing UV activation because there are no known paper whiteners or inks that fluorescence in the infrared spectrum. Thus, when IR fluorescent dyes are used, the detected signal only comes from inks intentionally put down and not from any inks or paper whiteners that are present in the paper. However, a disadvantage of these systems is that the amount of fluorescent IR light received from the dye is strongly influenced by the absorption of radiation by ink or other coloring material under the dye, which modifies the reflectivity of the surface and thus the amount of IR light detected by the photodiode.
In order to overcome this problem, a relatively complicated detection system has been proposed in which both stimulating laser radiation and IR fluorescent radiation are detected by two photodiodes, with appropriate optical filters over each diode to detect either the laser radiation or the fluorescent radiation. The outputs of the photodiode are processed in a ratio circuit to give a reliable signal that this corrected for variations in reflectivity caused by printing. The system in essence cancels any variation in reflectivity. However, the relationship between the incident laser light intensity and the intensity of the IR fluorescent light is not linear. Thus, when a ratio is taken, the measured ratio varies with the amount of light absorption by any ink under the dye. As a consequence, a complex electronic circuit must be used to give an artificial non-linear relationship between the input from the incident light photo cell and the input to the ratio determining circuit, to make the incident light response as non-linear as the IR fluorescent response. This is necessary to make the output respond linearly to the IR dye, despite variations in absorption resulting from underlying ink. However, this method is excessively complex and difficult to adjust. Moreover, the foregoing system is subject to interference from background illumination.
Systems for coding transparent receiver sheets for subsequent machine decoding have also been proposed. In commonly assigned U.S. Pat. No. 5,146,087, the disclosure of which is incorporated herein by reference, an imaging process utilizing invisible IR absorbing marking materials to form bar codes is disclosed. Because a transparent receiver sheet is used, the problem of a low contrast ratio resulting from the use of fluorescent compounds in the sheet does not arise.